1. Field of the Invention
This invention relates in general to magnetic recording, and in particular to apparatus and methods for reducing the level of record current necessary to effect recording in a magnetic medium.
2. Description Relative to the Prior Art
Various techniques have been suggested and/or adopted in the prior art to effect a reduction in the level of record head current necessary to effect recording in a magnetic medium. Among such techniques are (1) the use of cross-field recording, whereby a high frequency bias field is pumped into a magnetic medium by use of a separate bias-producing head in contact with the support, or back, side of a record medium, and (2) the use of a permanent magnet, proximate a record head, to provide DC bias for recording, as is practiced in a number of inexpensive tape recorders.
With the trend toward higher and higher track densities, record current levels in multitrack heads are a matter of major concern. Record currents which are too high, aside from causing the generation of excessive heat, have a tendency to cause inductive coupling between the discrete cores of multitrack heads, thereby undesirably creating core-to-core crosstalk. Cross-field recording alleviates these problems a bit by splitting the total current between two heads, one carrying the information signal and the other carrying the bias signal; but still the total current employed is high. The use of a permanent magnet, on the other hand, while lessening the total current required for recording (all things being equal), undesirably means DC biasing and may cause the pole tips of the signal-carrying record head to become magnetized.
In R. L. Wallace's definitive treatise appearing in Bell Systems Technical Journal, October 1951, page 1162, the field strength H at a distance Z from a recording medium of thickness .delta., which has been recorded along and in a direction X, with a signal of wavelength .lambda. is defined as: ##EQU1## Since B=4.pi.I.sub.m, where B is the flux density in gauss, ##EQU2## and since, by definition, ##EQU3##
Interpreting Equation (1), it will appear that, whereas a DC recording (.lambda.=.infin.) in a magnetic recording medium will have zero field outside the recording medium, an AC recording in such medium will be productive of a field above the recording surface which is wavelength-dependent.
There are currently available in the marketplace a number of recording media having high coercivity and high remanence. Although such media, pursuant to the below-described invention, are not employed for the recording of information signals therein, they are nonetheless employed as ancillary media to effect recording of information signals in primary media of lesser coercivity. In parts of this specification, 3M Metafine tape, available from the 3M Company, Minneapolis, Minnesota, is discussed as representative of a class of ancillary media useful in practicing the invention. The coercivity of 3M Metafine tape is 1130 oersteds (oe) and its saturation flux is 3600 gauss. Although 3M Metafine tape is specifically identified, any such medium which has a higher coercivity than a primary medium adapted to record information signals will suffice in the practice of the invention. In other words, relative coercivities, and not specific values of coercivities, are what are of concern.